This invention pertains to an inkjet ink, in particular to an aqueous inkjet ink comprising self-dispersible pigment and certain soluble polymers to improve print quality.
Inkjet printing is a non-impact printing process in which droplets of ink are deposited on print media, such as paper, to form the desired image. The droplets are ejected from a printhead in response to electrical signals generated by a microprocessor.
Both dyes and pigments have been used as colorants for inkjet inks. While dyes are typically easier to formulate compared to pigments, they tend to fade quickly and are more prone to rub off. Inks comprising pigments dispersed in aqueous media are advantageously superior to inks using water-soluble dyes in water-fastness and light-fastness of printed images.
Pigments suitable for aqueous inkjet inks are in general well-known in the art. Traditionally, pigments were stabilized by dispersing agents, such as polymeric dispersants or surfactants, to produce a stable dispersion of the pigment in the vehicle. More recently though, so-called “self-dispersible” or “self-dispersing” pigments (hereafter “SDP”) have been developed. As the name would imply, SDPs are dispersible in water without dispersants.
SDPs are often advantageous over traditional dispersant stabilized pigments from the standpoint of greater stability and lower viscosity at the same pigment loading. This can provide greater formulation latitude in final ink.
Prints made with SDP ink, however, tend to be susceptible to rub off and smear. EP-A-1114851 demonstrates (Comparative Example 2 in Table 3) the problem of poor smear resistance in an SDP inkjet ink (therein referred to as rubbing/scratching resistance). There is taught the combination of SDP and dispersant stabilized pigment to improve image properties. EP-A-1158030 likewise demonstrates (Example 9 in Table 1) the problem of poor smear resistance with SDP inkjet ink (therein is referred to as highlighter resistance).
Addition of polymer binder to improve print properties is often proposed. Ink with SDP and polymer are disclosed, for example, in U.S. Pat. No. 5,571,311, U.S. Pat. No. 5,630,868, U.S. Pat. No. 5,672,198, U.S. Pat. No. 6,057,384, U.S. Pat. No. 6,103,780, U.S. Pat. No. 6,329,446, US 20020147252, EP-A-1304364, EP-A-1146090 and EP-A-0894835.
All of the above-identified publications are incorporated by reference herein for all purposes as if fully set forth.
SDP ink, like any inkjet ink, must exhibit good jetting performance (jetability). Favorable jetability characteristics include stable drop volume and direction, and little or no kogation. Additives to the ink formulation to improve print quality should not come at the expense of jetability.
A need still exists for improved inkjet ink formulations of SDPs that provide good print quality and good jetability.